Replacing a Wooden Joist with RSJ Steel Beam – Step-by-Step Home Renovation Guide

Replacing a failing wooden joist with an RSJ steel beam is a common renovation challenge in older properties. Whether dealing with rot, insect damage, or inadequate strength for modern loads, steel beams offer a durable, long-lasting solution. This comprehensive guide explains when and how to replace timber with steel, with full consideration of structural engineering and Building Regulations.

When to Replace Timber with Steel

Signs Your Joist Needs Replacement

Structural damage:

• Visible rot or decay
• Woodworm or beetle infestation
• Cracks or splits in timber
• Excessive deflection or sagging
• Previous fire damage

Inadequate capacity:

• Floor bounce or vibration
• Historic undersizing for modern loads
• Change of use requiring higher capacity
• Removal of intermediate supports

Building alterations:

• Creating larger open spaces
• Adding new loads (bathroom, kitchen units)
• Converting attics to habitable rooms
• Strengthening for Building Regulations upgrades

Why Choose Steel Over Timber?

Advantages of RSJ replacement:

Strength: Steel typically 15-20 times stronger than timber for same depth, allowing:

• Longer spans without intermediate supports
• Support for heavier loads
• Thinner sections savingheadroom

Durability: Steel immune to:

• Rot and fungal decay
• Insect Attack (woodworm, beetles)
• Warping, twisting, shrinkage
• Provides 100+ year service life

Consistency: Manufacturing quality control ensures:

• Predictable properties
• No defects (knots, splits)
• Reliable calculations

Fire resistance: Properly protected steel performs well in fire:

• Intumescent coatings provide excellent protection
• Predictable performance under heat
• Often easier to achieve required fire ratings

Structural Considerations

Load Transfer

When replacing a timber joist with steel:

Understand what the joist supports:

• Floor boards and finishes above
• Ceiling below
• Partition walls running parallel
• Fixtures (baths, radiators, furniture)

Calculate total load:

• Dead load: Permanent construction elements
• Live load: Occupants, furniture, moveables
• Use Building Regulations minimums (1.5-2.0 kN/m² for floors)

Example calculation:

Existing timber joist: 225 × 50mm at 400mm centers, 4m span

• Floor area per joist: 4m × 0.4m = 1.6 m²
• Dead load: 0.50 kN/m² × 1.6 = 0.8 kN total = 0.2 kN/m on joist
• Live load: 1.5 kN/m² × 1.6 = 2.4 kN total = 0.6 kN/m on joist
• Total UDL: ~0.8 kN/m

Replacement steel:

• For 4m span, 0.8 kN/m load
• Even smallest RSJ (127×76×13) has capacity ~8 kN/m
• Massive overstrength! But minimum practical size for handling/connections

Typical choice: 152×127×37 RSJ - easy to handle, good bearing area, future-proof

Support Requirements

Steel beam needs proper bearing:

End supports:

• Masonry walls: Minimum 100mm bearing on padstones
• Timber frame: Steel posts or engineered timber posts
• Concrete: Direct bearing possible with DPC layer

Intermediate supports (if required):

• Steel columns on foundations
• Timber posts (properly sized)
• Load-bearing walls

Building Regulations Requirements

UK Compliance (2026)

You MUST:

1. Notify Building Control before work (or use approved inspector)

   • Fee: £200-350 typical
   • Multiple inspection visits

2. Engage structural engineer

   • Design replacement beam
   • Verify support adequacy
   • Produce calculations for Building Control
   • Cost: £250-500

3. Follow approved plans exactly

   • No improvisation permitted
   • Changes require engineer re-approval

4. Arrange inspections:

   • Before installation (temporary works)
   • After installation (before loading)
   • Final sign-off

Penalties for non-compliance:

• Enforcement action
• Requirement to expose work for inspection
• Potential demolition and rebuild
• Insurance invalidation
• Property sale complications

When Professional Installation Required

You CANNOT DIY:

• Load-bearing external walls
• Situations requiring complex temporary support
• Multi-story buildings
• If you lack competence/confidence

You CAN consider DIY (with engineer oversight):

• Internal floor joists in single-story situations
• Small spans (<3m)
• Light loads
• WITH proper temporary support
• WITH engineer-approved method

Recommended: Hire professional builder for structural work, DIY the finishing

Step-by-Step Replacement Procedure

Phase 1: Design and Approval (Weeks 1-4)

Week 1: Engage structural engineer

• Site visit to assess existing structure
• Measure spans, loads, support conditions
• Identify timber condition
• Discuss objectives

Week 2: Engineer produces design

• Calculate loads accurately
• Design replacement beam
• Specify supports and connections
• Produce drawings and calculations

Week 3: Submit to Building Control

• Engineer’s calculations
• Proposed method statement
• Site plans
• Application fee

Week 4-6: Await approval

• Building Control reviews submission
• May request clarifications
• Approval granted (or revisions requested)

Phase 2: Procurement and Preparation (Week 7)

Order materials:

• RSJ beam to exact specification (size, length, grade)
• Padstones (if required)
• Joist hangers or connection plates
• Temporary support equipment (acrow props, strongbacks)
• Fire protection materials
• DPC membrane

Lead times 2026:

• Standard size beams: 1-2 weeks
• Non-standard sizes: 3-4 weeks
• Fabricated connections: 2-3 weeks

Prepare site:

• Clear area above and below floor
• Protect finishes with dustsheets
• Arrange access for beam delivery
• Set up temporary lighting/power

Phase 3: Install Temporary Support (Week 8, Day 1)

Critical safety step - do not rush!

Below the floor:

1. Position strongbacks (100 × 50mm timber planking) perpendicular to joists

   • Usually 2-3 strongbacks needed
   • Span from wall to wall
   • Located 600mm each side of joist to be replaced

2. Install acrow props under strongbacks

   • One prop per meter of strongback length approximately
   • On scaffold boards to spread load
   • Adjust to just touch (no load yet)

3. Progressively load props

   • Tighten each prop quarter-turn
   • Move along line, repeating
   • Continue until floor is fully supported
   • You should be able to remove joist without any movement

Building Control Inspection Point 1: Verify temporary works adequate

Phase 4: Remove Timber Joist (Week 8, Day 1-2)

With temporary support in place:

1. Cut joist free from fixings

   • Use reciprocating saw or handsaw
   • Cut near each end (within bearing area)
   • Release any joist hanger fixings

2. Remove joist carefully

   • May need to cut into sections if long/heavy
   • Watch for hidden pipes/cables
   • Extract through floor above or below

3. Inspect joist pockets

   • Check wall condition where joist sat
   • Look for damp, rot spreading to wall timbers
   • Clean out debris

4. Create/enlarge joist pockets for steel

   • Steel likely taller than timber
   • May need to remove bricks/blocks
   • Ensure 100mm minimum bearing each end
   • Level and clean bearing surfaces

Phase 5: Install Padstones (Week 8, Day 2)

If required by engineer (usual for masonry):

1. Create recess for padstone

   • Depth = padstone thickness + 10mm mortar bed
   • Width × length per engineer’s specification

2. Mortar bed

   • 10mm thick 1:3 cement:sand mortar
   • Level carefully with spirit level

3. Set padstone

   • Lower into fresh mortar
   • Tamp to achieve perfect level
   • Verify both padstones at same height
   • Allow to cure 24-48 hours before loading

4. DPC layer

   • Place bitumen felt or plastic DPC on padstone
   • Prevents moisture transmission to steel

Phase 6: Install RSJ Beam (Week 8-9, Day 3-4)

Building Control Inspection Point 2: Before beam installation

Lifting and positioning:

1. Prepare beam

   • Apply rust inhibitor if not pre-treated
   • Mark center and end positions
   • Attach lifting straps

2. Lift into position

   • Use appropriate equipment for weight
   • Guide carefully into joist pockets
   • Lower onto padstones

3. Verify bearing

   • Minimum 100mm bearing each end
   • Check with tape measure
   • Beam square to walls

4. Level beam

   • Check level in both directions
   • Add steel packing shims if needed (never wood)
   • Aim for perfect level (critical for floors above)

5. Temporary support

   • Keep props in place under new beam initially
   • Add extra props directly under beam ends

Phase 7: Connect Floor Structure (Week 9, Day 4-5)

Two main options:

Option A: Joist hangers on steel beam

Best when floor joists run perpendicular to replacement joist:

1. Bolt joist hangers to steel flange

   • Use M12 or M16 bolts through flange
   • One hanger per floor joist
   • Position accurately to maintain joist spacing

2. Connect flooring joists

   • Insert joists into hangers
   • Nail/screw through hanger tabs
   • Verify all connections secure

Option B: Timber plate on top flange

Best when need continuous fixing surface:

1. Bolt timber to steel

   • 100 × 50mm treated timber along top flange
   • M12 bolts at 600mm centers through flange
   • Use washers under bolt heads

2. Fix floor joists to timber

   • Standard joist hangers to timber plate
   • Or notch joists over timber plate and skew-nail

Check level of entire floor before proceeding

Phase 8: Make Good and Finish (Week 9-10)

1. Fire protection (if required)

   • Intumescent paint (follow manufacturer’s specs)
   • Or plasterboard encasement
   • Building Regulations specify rating required

2. Below-floor boxing/ceiling

   • Plasterboard box around exposed beam
   • Or leave exposed for industrial aesthetic
   • Make good connections to existing ceiling

3. Above-floor finishing

   • Replace floorboards/chipboard
   • Ensure good fixing to joists
   • Level with existing floor

4. Progressively remove temporary support

   • Release props slowly and evenly
   • Monitor for movement
   • Remove strongbacks last

Building Control Inspection Point 3: Final inspection

Phase 9: Final Works (Week 10-11)

1. Decorating

   • Replaster damaged areas
   • Paint/decorate to match existing
   • Install new skirting if required

2. Final checks

   • Walk floor to verify no movement
   • Check ceiling below for cracks
   • Verify all fixings secure

3. Building Control sign-off

   • Arrange final inspection
   • Receive completion certificate
   • File safely for future property sales

Common Problems and Solutions

Problem 1: Beam Won’t Fit

Cause: Joist pockets too small or beam too large

Solution:

• Enlarge pockets carefully (masonry saw/chisel)
• Verify beam size matches engineer’s specification
• If beam genuinely oversized, contact supplier

Problem 2: Floor Still Bouncy

Possible causes:

• Temporary props not yet fully removed
• Connections not adequately fixed
• Other joists in floor also weak

Solutions:

• Verify all connections tight
• Remove all temporary support
• Check adjacent joists’ condition
• May need to stiffen entire floor with bridging

Problem 3: Ceiling Cracks Below

Cause: Movement during installation or inadequate temporary support

Solution:

• Fill cracks with flexible filler
• Monitor for continued movement (indicates ongoing problem)
• If worsening, cease work and contact engineer

Problem 4: Steel Beam Rusting

Cause: Moisture contact, no protection applied

Solution:

• Wire brush to remove loose rust
• Apply rust converter
• Paint with rust-inhibiting primer + topcoat
• Investigate moisture source and eliminate

Cost Breakdown Example

Typical single joist replacement project (4m span):

Item	Cost
Structural engineer	£300-450
Building Control fee	£220-320
152×127×37 RSJ, 4.3m	£225-280
Two padstones	£60-90
Joist hangers (8×)	£60-100
Acrow props hire (4, 2 weeks)	£80-140
Fixings, DPC, misc materials	£60-100
DIY Total (materials + fees)	£1,005-1,480
Professional installation labor	£600-1,000
Professional Total	£1,605-2,480

Savings from DIY finishing work: £300-500 Net cost if hiring pro for structural, DIY finish: £1,300-2,000

Conclusion

Replacing a Timber joist with an RSJ steel beam is a complex structural alteration requiring careful planning, engineering design, and Building Regulations compliance. While competent DIYers can undertake some aspects, critical structural work should involve professionals to ensure safety.

Success factors:

1. Engage structural engineer early - don’t skip this!
2. Obtain Building Control approval before starting
3. Use adequate temporary support - never rush this step
4. Install beam to exact specifications
5. Allow proper curing times before removing support
6. Arrange all required inspections

Consider professional installation if:

• Spans >4m
• Multi-story building
• Complex load paths
• You lack confidence or experience
• Access is difficult

The cost of professional design and installation is far less than repairing structural failure or dealing with Building Control enforcement action.

Disclaimer: This guide provides general information only. All structural alterations must be designed by a chartered structural engineer and approved by Building Control. Never undertake structural work without professional involvement and proper approvals.